Tissue injury or stress triggered biochemical inducers of oxidative stress causing the production of free radical molecules may ultimately manifest in severe pathologies including cardiovascular diseases, diabetes, cancer, autoimmune disease, and end-stage organ failure. Macrophages and neutrophils and their proinflammatory cytokines, tumor necrosis factor (TNF-α), interleukin-1β, and interferon-γ increase mitochondrial and nicotinamide adenine dinucleotide phosphate oxidase-generated reactive oxygen and nitrogen species (RONS) that initiate the innate immune cascade.1 In general, the generation of RONS is counterbalanced by the activity of antioxidant enzymes and other redox molecules. However, excess reactive oxygen species (ROS) concentrations can be detrimental and may lead to cellular injury damaging DNA, lipids, and proteins. There is a significant evidence now showing that oxidative stress as a result of exposure to RONS is associated with lung pathologies, including asthma, chronic obstructive pulmonary disease, and ozone-induced lung injury that lead to an increase in the mutagenic base by-product 7,8-dihydro-8-oxoguanine (8-oxoG). This genomic 8-oxoguanine DNA glycosylase (OGG1) is the primary DNA repair enzyme responsible for the base excision repair of 8-oxoG. Recent studies have shown that OGG1 plays a central role in the maintenance of genome integrity and host homeostasis through either lesion repair or elimination of cells with malignant potential depending on the magnitude of guanine oxidation.2
Counterintuitively though, Visnes and coworkers have recently published data in Science3 showing that a novel small-molecule inhibitor that binds to the active site of OGG1 and blocks recognition of 8-oxoG in DNA, exhibits anti-inflammatory properties. Oxidized guanines arise in proinflammatory gene promoters that facilitate loading of nuclear factor κB (NF-κB) transcription factor binding responsible for downstream gene expression of proinflammatory chemokine and cytokine genes.4 Visnes et al3 screened for OGG1 inhibitors and developed TH5487, a potent inhibitor of TNF–induced inflammatory responses in cultured mouse and human lung epithelial cells. Most notably, TH5487 inhibited TNF-induced neutrophilic inflammation in an in vivo mouse model. The authors also demonstrated that TH5487 prevented OGG1 recognition of G-rich regions adjacent to NF-κB-binding sites in promoters of proinflammatory genes, thus representing an entirely new strategy to control inflammation (Figure 1).
Central to this study has been the hitherto underappreciated role of TH5487 as an epigenetic modulator of the physical chromatin topology that is adjacent to the areas of DNA lesions. For example, immobilized OGG1-green fluorescent protein fusion proteins at genomic DNA lesions that were treated with TH5487 increased the nuclear mobility of OGG1-green fluorescent protein, suggesting that TH5487 prevented OGG1 binding to its genomic substrate in living cells. Because major epigenetic events involve the complexity of methylation and acetylation of histones and regulatory factors, DNA methylation, and small noncoding RNAs, TH5487 control on the kinetic regulation of chromatin DNA will need future attention. In addition to perturbation of NF-κB in TNF-α-exposed cells, it will be interesting to see whether TH5487 perturbs DNA occupancy of other OGG1-dependent transacting factors in a more global manner. Likely, 8-oxoG located in the linker DNA of the dinucleosome is removed by OGG1 when global chromatin is accessible in an open configuration. At the same time, heterochromatinizaton by TH5487 may cause failure of NF-κB loading and subsequent silencing of proinflammatory genes (Figure 1). Indeed, we recently reported a similar silencing mechanism where global tertiary interactions between DNA transcription factor (IRF1) and RNA protein Lv1 (an isoform of hnRNP L) caused cotranscriptional platforms of alternative splicing and transcriptisome assembly to silence CEACAM1, an anti-inflammatory molecule expressed on leukocytes.5 Understanding how small molecules including TH5487 disrupt DNA occupancy could lead to the development of new classes of drugs with anti-inflammatory capacities.
Visnes and coworkers3 also showed that TH5487 impressively decreased neutrophil accumulation in bronchoalveolar lavage fluids, implying the anti-inflammatory potential of this novel compound. Indeed, the inflammatory gene modulation and neutrophil regulation by TH5487 shown in this study is robust; therefore, one may envision the use of TH5487 early after transplantation. A potential tumorigenesis due to lack of DNA repair discourages prolonged use of this inhibitor. Moreover, the entirely new strategy to control inflammation gives rise to many questions and additional thoughts. (1) Supplementation of TH5487 in this study had almost no impact on the number of macrophages in bronchoalveolar lavage fluid (potentially related to the short observation period of 16 h). OGG1 deficiency inhibited TNFα expression in splenic macrophages,6 whereas others reported that OGG1 knockout in macrophages increased mtDNA oxidization and enhanced IL-1β secretion augmenting atherosclerosis.7 Because OGG1 may regulate macrophages in a disease-specific manner, utilization of TH5487 transplant models awaits further studies. (2) Major surgeries of organ transplantation are linked to a postoperative insulin resistance, a well-recognized factor for surgical complications including infections or delayed wound healing. Because recent studies using OGG1 transgenic8 mouse have established that OGG1 is essential for insulin-tolerability, putative insulin resistance derived by OGG1 inhibition will need to be addressed. (3) Recent studies reported on protective functions of a fusion protein construct targeting OGG1 to mitochondria.9 Apart from its anti-inflammatory impact, the effects of OGG1 inhibition on graft parenchymal cells (eg, hepatocytes) need future attention. (4) ROS produced mainly by innate immune cells have long been considered solely detrimental in organ transplantation. Recent evidence, however, revealed that ROS can limit proinflammatory innate immune cell phenotype, causing neutrophil apoptosis and inhibiting cytokine production.10 As OGG1 has the capacity to repair ROS-induced DNA damage on G-rich promoters while enhancing inflammation, ROS and OGG1 seem to cooperate in the modulation of inflammation. Clearly, the seemingly context-dependent role of OGG1 in the maintenance of genome integrity and homeostasis versus proinflammatory gene programs suggests this area should be explored further.
Nevertheless, the findings of Visnes et al3 hint at novel molecular interactions that occur in highly complex, interconnected, and dense microenvironments. The influence of accessibility, diffusion, enzyme structure, and free energy of chemical reactions will be relevant in the search for alternative anti-inflammatory modalities to control organ damage following transplantation and other inflammatory diseases.
1. Yang D, Elner SG, Bian ZM, et al. Pro-inflammatory cytokines increase reactive oxygen species through mitochondria and NADPH oxidase in cultured RPE cells. Exp Eye Res. 2007;85:462–472.
2. Wang R, Li C, Qiao P, et al. OGG1-initiated base excision repair exacerbates oxidative stress-induced parthanatos. Cell Death Dis. 2018;9:628.
3. Visnes T, Cázares-Körner A, Hao W, et al. Small-molecule inhibitor of OGG1 suppresses proinflammatory gene expression and inflammation. Science. 2018;362:834–839.
4. Pan L, Zhu B, Hao W, et al. Oxidized guanine base lesions function in 8-oxoguanine DNA glycosylase-1-mediated epigenetic regulation of nuclear factor κb-driven gene expression. J Biol Chem. 2016;291:25553–25566.
5. Dery KJ, Silver C, Yang L, et al. Interferon regulatory factor 1 and a variant of heterogeneous nuclear ribonucleoprotein L coordinately silence the gene for adhesion protein CEACAM1. J Biol Chem. 2018;293:9277–9291.
6. Seifermann M, Ulges A, Bopp T, et al. Role of the DNA repair glycosylase OGG1 in the activation of murine splenocytes. DNA Repair (Amst). 2017;58:13–20.
7. Tumurkhuu G, Shimada K, Dagvadorj J, et al. Ogg1-dependent DNA repair regulates NLRP3 inflammasome and prevents atherosclerosis. Circ Res. 2016;119:e76–e90.
8. Komakula SSB, Tumova J, Kumaraswamy D, et al. The DNA repair protein OGG1 protects against obesity by altering mitochondrial energetics in white adipose tissue. Sci Rep. 2018;8:14886.
9. Hashizume M, Mouner M, Chouteau JM, et al. Mitochondrial-targeted DNA repair enzyme 8-oxoguanine DNA glycosylase 1 protects against ventilator-induced lung injury in intact mice. Am J Physiol Lung Cell Mol Physiol. 2013;304:L287–L297.
10. Harbort CJ, Soeiro-Pereira PV, von Bernuth H, et al. Neutrophil oxidative burst activates ATM to regulate cytokine production and apoptosis. Blood. 2015;126:2842–2851.
Central to this study has been the hitherto underappreciated role of TH5487 as an epigenetic modulator of the physical chromatin topology that is adjacent to areas of DNA lesions.